When plants encounter an environment where the temperature exceeds the upper limit or the lower limit of an optimal temperature for growth or germination, what is called temperature stress factor, physiological functions of cells decline slowly or rapidly and thus various disorders may arise. While it has been known that phytohormones and some chemical substances such as plant growth regulators have an effect of reducing temperature stress of plants, these substances cannot be said to be sufficient in terms of their effects. It has also been known that 4-oxo-4-[(2-phenylethyl)amino]-butyric acid and derivatives thereof have activity to promote growth of roots (see, for example, Japanese Patent Publication No. 4,087,942, Japanese Unexamined Patent Publication No. 2001-139405, and Plant and Soil, 255: 67-75, (2003)). However, an effect of reducing temperature stress of plants has never been known.